Breast cancer and other breast lesions continue to be a significant threat to women's health. X-ray mammography currently is the most widely used tool for early detection and diagnosis, and is the modality approved by the U.S. Food and Drug Administration to screen for breast cancer in women who do not show symptoms of breast disease. A typical x-ray mammography system compresses and immobilizes a patient's breast on a breast platform positioned between an x-ray source and an x-ray imager, and takes a projection x-ray image (called here a conventional mammogram or simply mammogram) using a collimated cone or pyramid beam of x-rays at appropriate factors such as mA (current), kVp (voltage) or keV (energy), and mS (exposure time). In the United States, typically two views are taken of each breast, one from above (cranial-caudal, or CC, generally at a 0° angle—with the breast platform essentially horizontal and the x-ray beam axis normal to the platform) and one from the side (mediolateral-oblique, or MLO, at an angle of typically around 45°). Different typical views may be taken in other countries. The x-ray source typically is an x-ray tube operating at or in the neighborhood of 25 kVp, using a molybdenum or rhodium rotating anode with a focal spot of about 0.3–0.4 mm and, in some cases, 0.1 mm or less. An anti-scatter grid between the breast and the imager can be used to reduce the effects of x-ray scatter. Separately for each view, the breast is compressed to reduce patient motion and also for reasons such as reducing scatter, separating overlapping structures in the breast, reducing the x-ray thickness of the imaged breast and making it more uniform, and providing more uniform x-ray exposure. Traditionally, the imager has been a film/screen unit in which the x-rays impinging on the screen generate light that exposes the film. Currently, electronic digital flat panel array receptors are replacing the film/screen units in mammography systems. A Selenia™ digital mammography system with such a digital flat panel x-ray receptor or imager is offered by Lorad, a division of the assignee hereof, Hologic, Inc. of Bedford, Mass. Digital mammography has significant advantages and in time may fully supplant film/screen systems. Additional information regarding digital mammography systems and processes offered by the common assignee can be found at <www.hologic.com>.
Mammograms, whether from film/screen units or from digital flat panel imagers, are difficult to read, and the challenge of false negatives and false positives continues to be significant. Many advances have been made in recent years in image acquisition and in image processing, but a need still remains to reduce the rates of false negatives and positives, at least in screening mammography. Additional information can be gained through modalities such as CT and MRI, but examination and interpretation time, and cost and other factors, have limited their use in screening for breast cancer. Ultrasound breast examination has been proposed as an adjunct to x-ray examination, providing synthesized ultrasound images of thick slices of the breast that simulate the same projection view as an x-ray view and are displayed together with the x-ray view, and a unit taking both x-ray and ultrasound images has been proposed and is believed to be available at least for clinical testing. See, e.g., Patent Application Publication No. U.S. 2003/0007598 A1 and U.S. Pat. No. 5,983,123. Further, digital tomosynthesis has been proposed for x-ray breast imaging, and a laboratory unit is believed to have been installed in the U.S.A., as reported in Wu, Tao, 2002, Three-Dimensional mammography Reconstruction Using Low Dose Projection Images, PhD thesis, Brandeis University, incorporated here by reference. The assignee hereof has exhibited a breast tomosynthesis system at a trade show in November 2003 and November 2004, and has carried out clinical testing. See, also, Patent Application Publication No. 2001/0038681 A1 and PCT application International Publication No. WO 03/020114 A2 published Mar. 13, 2003, both incorporated herein by reference. Digital tomosynthesis in more general contexts also has been proposed. See, e.g., U.S. Pat. Nos. 6,885,724, 6,289,235 and 5,051,904, commonly assigned U.S. Pat. No. 4,496,557 and published patent applications US 2004/0109529 A1, US 2004/0066884 A1, US 2005/0105679 A1, US 20050129172A1, and Digital Clinical Reports, Tomosynthesis. GE Brochure 98-5493, 11/98, all incorporated herein by reference. Reference markers can be used in x-ray imaging for purposes such as checking the rotation angle and unwanted shift of center of rotation of an x-ray source and receptor (imager), and fiducial phantoms can be used in 3D angiography to calibrate for irregular scan geometries. See, e.g., U.S. Pat. Nos. 5,051,904, 5,359,637, and 6,289,235, N. Navab, et al., Dynamic geometrical calibration for 3D cerebral angiography, SPIE Vol. 2708, pp. 361–370, and said PCT published application WO 03/020114 A2, all incorporated by reference here.
A tomosynthesis system specifically for imaging patients' breast is disclosed in commonly owned published U.S. patent application No. US2004/0101095A1, which is hereby incorporated by reference here. In known tomosynthesis breast units, it is believed that the motion of the x-ray source typically is symmetrical relative to a line from the focal spot of the x-ray source to the breast platform and perpendicular to the platform. For example, the source generally moves through an angle of ±15° or ±30°, where 0° corresponds to the position for the CC or MLO mammogram view. One of the references identified above, U.S. Pat. No. 6,885,724, discusses asymmetric geometry in the context of a table or wall configuration where the room dimensions may preclude a fully symmetrical scan, and proposes modifying the typically symmetrical scan such that the sweep on one side of a 0° position is not the same as the sweep on the other side, so as to accommodate room size limitations.